1. Field of the Invention
The present invention generally relates to optical lenses and projection systems, and more particularly to an achromatic, Fresnel lens which may be used in a low-profile overhead projector.
2. Description of the Prior Art
Transmissive overhead projectors (OHP's) are known in the art, and are generally comprised of a base having a transparent stage area, a light source inside the base, a projection head mounted above the stage, and a condensing lens system located near the stage for collecting and directing the light towards the projector head. The condensing lens system often takes the form of a Fresnel lens or a two-element Fresnel lens combination, as depicted in U.S. Pat. No. 4,436,393.
Several attempts have been made to decrease the base height of OHP's, for portability and reduced obstruction of the projected image during use. To achieve such a low-profile base, some machines collapse during storage or transportation, then expand open for actual use. Machines of this type are described, for example, in U.S. Pat. No. 4,969,733 and German Patent Application No. 3,519,506. These collapsible OHP's have several disadvantages, however, such as requiring additional parts which increase the complexity of the machine and add to its cost; the moving parts may also adversely affect alignment of the optical elements over time. Finally, there is no base height advantage of these machines over conventional OHP's when in operation.
Other attempts have been made to construct a low base height OHP without the need to collapse the base and optical elements. For example, U.S. Pat. No. 4,741,613 employs a three-element refracting Fresnel lens system to allow the point light source to be very close to the stage. There are several aspects of this invention, however, which could be improved. First, the requirement of three Fresnel lenses necessarily adds to the cost. Secondly, this system does not adequately compensate for the diminished illumination near the periphery of the stage area. Finally, the dispersion (chromaticity) of a conventional Fresnel lens may lead to coloration of the projected image, particularly around the periphery thereof. Most OHP's locate the light source within the base to minimize this effect, but it is noticeable any time the projection head is moved from its optimum position. For example, if the projection head must be lowered to magnify an image or focus it on the projection screen, less red light at the periphery of the stage is captured by the projection lens, leading to a bluish tint at the border of the projected image. Similarly, moving the projection head up (away from the stage) can cause the border to have a reddish tint. This effect (which is even more pronounced in the three-element system of the '613 patent) can be overcome by repositioning the lamp within the base (referred to as "color tuning"), but this involves further user manipulation of the OHP and still requires a subjective judgment regarding the chromaticity of the projected image. Color tunability also adds to the cost of the OHP since it requires a mechanism for repositioning the lamp.
The OHP's disclosed in U.S. Pat. Nos. 4,118,761 and 4,952,053 use folded optical paths to provide a more compact base. In the '761 patent, the light source is also "off-axis," meaning that the apparent location of the light source does not coincide with the center of the stage, i.e., the apparent location is displaced from the normal to the stage center. That device requires a complex optical system including a parabolic reflector to provide collimated light, two planar grooved reflectors, and the condensing lens assembly; it may exhibit a slightly darkened edge, furthest from the light source. In the '053 patent, the darkened edges are compensated for by providing two light sources. The twisted and curved mirrored, grooved prisms are also more difficult to fabricate than flat condensers. Finally, folded optical paths may create problems with full-size stage formats, such as European (A4) styles, since extremely tight folds such as are necessary to achieve a low-profile may cause the light source itself to interfere with the folded light path.
The OHP of the present invention overcomes several of the above limitations by using a novel catadioptric Fresnel lens. While the use of any catadioptric lens in an OHP is in itself novel, catadioptric lenses are known in other art areas. The term "catadioptric" refers to a lens which uses both reflection and refraction to redirect or bend light waves. See, e.g., U.S. Pat. Nos. 2,003,804, 4,859,043 and 5,097,395. As depicted in those patents, catadioptric lenses are useful in collimating light, similar to Fresnel lenses. In U.S. Pat. No. 5,150,966, a catadioptric lens is used to adjust light intensity distribution by discarding light. See also U.S. Pat. No. 4,755,921, which describes a single element catadioptric lens having increased efficiency at high entrance angles, thereby providing a "fast" lens (low f-number) that is suited for compact optical devices.
Chromatic aberration is also a problem with catadioptric Fresnel lenses. The dispersion is caused by the optical properties of the lens material, i.e., its index of refraction varies with the wavelength of the light passing therethrough. There are several techniques, however, for minimizing this effect. For example, attempts have also been made to minimize chromatic aberration in single element lenses, such as by providing discontinuous axisymmetric surfaces, or by using aspheric surfaces. Diffraction gratings may be placed on the lens, including on a Fresnel lens, as depicted in U.S. Pat. No. 5,161,057. See also U.S. Pat. No. 4,337,759, which describes a curved base, catadioptric Fresnel lens with a controlled amount of chromatic dispersion for use as a solar concentrator. Many of the foregoing techniques minimize dispersion only for selected wavelengths, as opposed to the full (visible) spectrum. Most of the techniques (including the '759 patent) also require materials having specific indices of refraction, to achieve a specific focal length with color correction.
In light of the foregoing, it would be desirable and innovative to design a Fresnel lens which is usable in a low-profile OHP. The lens would advantageously be achromatic and constructed of any number of light-transmissive materials having a large range of refractive indices, and efficiently focus the light to the projection lens of the OHP. The condensing system preferably would avoid the use of micro-precision diffraction gratings, and would generally minimize the number of optical elements (such as grooved mirrors), including the number of elements in the condensing lens, but still be compatible with a folded optical path. Provision of such an achromatic lens would greatly reduce or minimize the need for color tuning.